Method for obtaining thin, light and rigid metal parts

ABSTRACT

The invention concerns a method for producing parts made of thin, light and rigid metal alloy essentially having the following steps: producing a core having the part shape; producing cavities in the core; producing shells made of metal alloy combined with reinforcing fibers with high modulus of elasticity; densifying the shells; and diffusion welding of the shells on the core by compression at temperature and pressure conditions for isothermal forging of the metal alloy used.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The invention relates to a process for manufacturing metal parts with asandwich structure. The metal parts are reinforced by fibers having ahigh elastic modulus and are joined together by diffusion bonding.

2. Description of the Related Art

Thin and lightweight parts made of metal alloys having a highstrength/mass ratio, i.e. mainly aluminum, magnesium and titaniumalloys, are used in aeronautics. In the case of turbomachines, theseparts are typically casings, casing arms and blade assemblies. However,such alloys have the drawback of having a low Young's modulus and theparts made with these alloys must consequently be strengthened by ribsso as to provide sufficient rigidity. The presence of these ribs howeverhas the drawback of increasing the mass of the part. In addition, theseribs may have complex shapes and it may consequently become veryexpensive to produce them.

Composites consisting of reinforcing fibers embedded in a metal matrixare also used at the present time in aeronautics, the fibers possiblybeing made of silicon carbide (SiC), of boron or of carbon and thematrix made of aluminum, magnesium and titanium alloy. Such materialshave mechanical properties that are substantially improved over the samematrix alloy used alone. By way of example, if a composite consisting ofsilicon carbide fibers embedded in a Ta6V titanium alloy matrix iscompared with the Ta6V titanium alloy used alone, it is found that themechanical strength is increased by 120%, the Young's modulus isincreased by 100% while the density is reduced by 15%.

Metal matrix composite are obtained essentially by strongly compressing,at the superplastic forming temperature of the metal alloy, a preformconsisting of reinforcing fibers and a metal alloy, the fibers possiblybeing woven or wound and the metal alloy possibly being in the form offoils placed between the fibers, in the form of a coating applied aroundthe fibers by the process called “physical vapor deposition” or “PVD”,the metal alloy also possibly being applied by plasma spraying onto thewoven or wound fibers. Hot pressing may be carried out in a die in apress when the shape of the part allows this pressing, that is to saywhen it has a predominantly plane shape. Otherwise, the pressing canalso be carried out in an autoclave, the part then being surrounded by acontainer, that is to say a sealed metal shell in which a vacuum iscreated, the part also possibly being pressed against a former. Suchprocesses allow thin composite parts to be produced which have improvedmechanical properties compared with the same part made of metal alloy.However, the use of these processes for producing large parts throughouttheir thickness would require the use of a large quantity of fiber,whereas only the fibers at the surface of the part are contributing tothe stiffness of this part according to a principle well known in thestrength of materials. Thus, because the cost of purchasing and of usingthese high-strength fibers is very high, the manufacturing cost of suchparts would be prohibitive.

Hybrid parts are also produced to comprise a fiber/metal alloy compositepart and a part made of metal alloy alone. To manufacture such pieces, ablank of the second part is machined and the first and second parts arepressed together using the aforementioned general process, this hotpressing bonding the two parts together by mutual diffusion of the alloyof each part into the other part.

In general, the constructing and the hot pressing of a part made of acomposite, comprising reinforcing fibers having a high elastic modulusembedded in a matrix made of a metal alloy, remain difficult operationssince these fibers cannot withstand large curvatures without breaking,because of their high elastic modulus. Since the pressures required bothfor the densification and for the diffusion bonding are very high, inorder for the fibers not to break, the following conditions are usuallysatisfied:

the fibers are arranged uniformly in plies, one beside another in aparallel manner;

during densification, the process must allow the matrix to flow veryhomogeneously around the fibers so as not to cause, due to the effect ofthe pressure, localized displacements of the fibers in which there wouldbe a risk of breaking them.

By way of example, the pressing and diffusion bonding of a compositeconsisting of reinforcing fibers made of silicon carbide with a matrixmade of Ta6V titanium alloy require a pressure of 600 to 800 bars at atemperature of about 900° C.

SUMMARY OF THE INVENTION

The invention provides process for producing thin and rigid metal parts,said process comprising in particular the following operations:

production of a core made of a metal alloy;

application, to each face of the core, of a shell made of a compositewhich includes reinforcing fibers embedded in a metal alloy, said fibershaving an elastic modulus at least equal to four times that of the metalalloy;

densification of the shells by pressing at least in the thicknessdirection at the superplasticity temperature of the metal alloysurrounding the fibers; and

diffusion bonding of the shells to the core by pressing at least in thethickness direction at the diffusion temperature of the metal alloys ofthe shells and of the core.

Such a process is noteworthy in that:

a plurality of emerging cavities is produced in the core on at least oneface of the core, for example by drilling, by electrical dischargemachining or by punching, with a lowest possible volume fraction of thecore, for example 0.9, the volume fraction of the core being the ratioV/V_(c) in which V_(c) is the volume of the solid core and V is thevolume of the remaining matrix of the core after the cavities have beenproduced, said cavities being uniformly distributed over the part; and

the shells are densified to the shape of the core before they areapplied to the core.

This process has the effect of forming, in the part of the cavitiesclosed at least on one side by the shells with negligible creep of saidshells into said cavities, and has the result of simultaneouslylightening and stiffening the parts, without increasing their thickness.

The curvature of the reinforcing fibers is kept approximately constantabove each cavity and in the vicinity of each cavity, therebysimultaneously preventing the fibers from breaking or allowing only anegligible proportion of them to break, and permitting these fibers tobe maintained in the best position so as to strengthen and stiffen thepart.

Thus, contrary to what may have been thought, it is possible to press,under the aforementioned conditions and without appreciable creep, intothe cavities of the composite shells which consist of reinforcing fibershaving a high elastic modulus embedded in a matrix made of a metalalloy, onto a core which itself comprises a multitude of cavities openat its surface, the prior densification of the shells making said shellsstiff enough to limit their creep into the cavities to negligiblevalues.

Advantageously, the metal alloys will be taken from the group comprisingtitanium, aluminum and magnesium, and the reinforcing fibers from thegroup comprising silicon carbide, boron and carbon, so as to combine alight metal alloy with reinforcing fibers having a high strength and ahigh elastic modulus.

In a first embodiment of the invention, the diffusion bonding is carriedout in a die in the press, for example with a heating die or with afurnace press. Such an arrangement has the effect of keeping the averagethickness of material between the cavities at a sufficient valuecompatible with the pressing process employed and has the result ofpreventing the core from collapsing during the pressing in a die.

In a preferred method of implementing the process, the shells are bondedto the core by isostatic pressing in an autoclave. The width of thecavities must then be limited to a value compatible with this type ofpressing. The present process can then be applied to parts which areimpossible to produce in a die, for example turbomachine casings. Itwill be understood that the pressure applied to the part by a fluid, inthis case the gas of the autoclave, favors creep of the shells into thecavities. However, it has been found that this creep may be regarded asbeing negligible when the dimensions of the cavities remain less than acertain limit which depends on the properties of the composite shell,thereby allowing the manufacture of parts under these conditions ofaeronautical or aerospace quality.

Advantageously, a minimum volume fraction of the core will be used so asto reduce the mass of the core and to lighten the parts of the samestiffness and strength.

Also advantageously, the volume fraction of the core V/V_(c) will beincreased in the vicinity of the regions for fastening the part. Thishas the effect of increasing the compressive strength of the part atthese points and has the result of allowing the part to be bolted withhigh tightening torques. In one particular embodiment, the core will besolid in the immediate vicinity of said fastening members.

Advantageously, cavities touching each other may be machined in the corealong suitable lines. This has the effect of forming ducts between theshells and has the result of allowing fluid to flow into the thicknessof the part. This result is particularly beneficial in the case ofstructural parts of a turbomachine, such as the casings and the casingarms: it is thus possible to distribute lubricant, fuel or gas atvarious temperatures, especially in order to control the operatingclearances.

Advantageously, these cavities touching one another each emerge only onone side of the core so as to maintain the cohesion of said core duringproduction of the part.

Advantageously, when the part is a turbomachine blade assemblycomprising a blade and a root at one end, the core extending into theblade and into the root, cavities with a reduced volume fraction of thecore V/V_(c) will be made in the core of the blade and, optionally,cavities with a high volume fraction of the core V/V_(c) will be made inthe root, thereby making it possible to produce very lightweight bladeassemblies which will be able, however, to withstand high root embedmentstresses. Advantageously, the blade assembly will be produced in a die,thereby allowing small volume fractions of the core and therefore aconsiderable weight saving.

DESCRIPTION OF THE DRAWINGS

The invention will be better understood and the advantages that itprovides will be more clearly apparent in the light of a detailedembodiment and of the appended figures.

FIG. 1 illustrates, in a sectional view, a casing wall.

FIGS. 2 and 3 illustrate possible shapes of the cavities machined in thecore.

FIG. 4 illustrates in a graph the results obtained in the case ofsilicon carbide fibers and of a Ta6V titanium-based metal alloy.

DETAILED DESCRIPTION OF THE INVENTION

Reference will firstly be made to FIG. 1. A part 1 is thin and comprisesa core 2 which is also thin and bounded in its thickness direction bytwo faces 3. The core 2 has a plurality of cavities 4 each emerging onat least one of the faces 3 of the core 2. In this example, the cavities4 each emerge on both faces 3 of the core 2. A shell 10 isdiffusion-bonded to each of the faces 3 of the core 2. Inner faces 11 ofthe shells 10 are the places where the diffusion bonding takes place.This diffusion bonding consists of a diffusional interpenetration of thematerial of the core 2 and of the shells 10 at the faces 3 and 11, whichare in mutual contact, with the core 2 and the shells 10, respectively.It will be understood that the cavities 4 are closed by the shells 10.In practice, these cavities 4 will be cylindrical. In the example inFIG. 2, these cavities 4 have a circular cross section and are arrangedin a staggered manner along approximately parallel lines 15, thedistance d between a cavity 4 and each of its neighbors beingapproximately constant, at least locally, that is to say in a limitedregion. In the example in FIG. 3, the cavities 4 are triangular andplaced back to back along approximately parallel lines 15, the verticesof the triangles of one line 15 being inverted with respect to thevertices of the triangles of the two neighboring lines 15, the distanced between the vertices and/or the sides of a triangle and of all theneighboring triangles being approximately constant. Moreover, thevertices of the triangles are rounded so as to reduce stressconcentrations that could occur in the material of the core 2.

In this example, the core 2 is made of Ta6V titanium-based metal alloyand the shells are made of a composite consisting of silicon carbidereinforcing fibers embedded in a matrix also made of Ta6V titanium-basedmetal alloy.

The process is as follows:

Manufacture of the core 2 in the shape of the part 1, takes place forexample by rolling, forging or machining; production of the cavities 4occurs directly in the casting, or by drilling, punching or electricaldischarge machining, the cavities 4 possibly being uniformly distributedover the part 1 and/or interrupted so as to reinforce locally the core 2when there are, for example, points of application of loads or bosses;machining and pickling of the faces 3 of the core 2 then take place.

Manufacture of the shells 10 made of a metal matrix composite by a hotpressing operation (hot isostatic compacting or uniaxial pressing in apress, if necessary in a die), under temperature and pressure conditionsallow this material to be densified using one of the standardtechniques: fiber plus foil, reinforced monolayers, prepregs obtained byplasma, winding of fibers coated by PVD (Physical Vapor Deposition), oranother equivalent process. Machining and pickling of the inner face 11of the shells 10 then take place. These shells 10 have the requirednumber of fiber layers for obtaining the mechanical strength andstiffness desired.

In order to join the shells 10 and the core 2 together by diffusionbonding, it is necessary:

to machine the faces 3 of the core 2, if necessary;

to clean, chemically pickle and rinse the faces 3 of the core 2 and theinner faces 11 of the shells 10, so as to prepare them for diffusionbonding; and

to join the shells 10 to the core 2 and to place the assembly in apressing tool or in a container capable of hot isostatic pressing in anautoclave, and to press it while complying with the pressure andtemperature cycles appropriate to the alloys of which the core 2 and thematrix of the shells 10 are composed so as to diffusion-bond the shells10 to the core 2.

Reference will be now be made simultaneously to FIGS. 1 and 4. Firstly,t_(o) will denote the thickness of the part 1, t_(c) the thickness ofthe core 2, t_(s) the thickness of each shell 10, with the equationt_(o)=2t_(s)+t_(c). In addition, K will denote the stiffness of the partobtained with the preset process, K₀ is the stiffness of this same part1 in monolithic form, that is to say all metal and without cavities 4, Mis the mass of the part obtained with the present process and M₀ is themass of the monolithic part. Finally, V_(c) will denote the total volumeof the core 2, V is the volume of material of the core 2 remaining afterthe cavities 4 have been produced and V/V_(c) is the volume fraction ofthe core.

The x-axis of the graph shows the thickness fraction of the shells 10,i.e. 2t_(s)/t_(o), this fraction obviously only varying between 0 and 1.

Curves 20 and 21 show the variations in the ratios K/K₀ and M/M₀,respectively, as a function of the thickness fraction of the shells2t_(s)/t_(o) for a volume fraction of the core V/V_(c)=1, that is saywithout cavities 4. Curves 22 and 23 show these same ratios for a volumefraction of the core V/V_(c)=0.75 and curves 24 and 25 show these sameratios for a volume fraction of the core V/V_(c)=0.50.

Curve 20 shows that the stiffness ratio K/K₀ may reach the maximum value2 when the part is fibrous over its entire thickness, that is to saywhen the thickness fraction of the shells 2t_(s)/t_(o) is equal to 1. Itis worthwhile pointing out that the stiffness ratio K/K₀ remains at 1.85and that the corresponding mass ratio M/M₀ given by curve 21 drops to0.94 when the thickness fraction of the shells 2t_(s)/t_(o) drops to0.25. In other words, although each shell 10 occupies only 12.5% of thethickness of the part 1, the stiffness of the part is increased by 85%and its mass reduced by 6%.

It is worthwhile pointing out also that, by virtue of curves 24 and 25,the stiffness of the part is increased by 80% and its mass reduced by33% when the core 2 is lightened by 50% and that each shell 10 occupies12.5% of the thickness of the part 1, corresponding respectively tovalues of K/K₀=1.8, M/M_(O)=0.67, V/V₀,=0.5 and 2t_(s)/t_(o)=0.25.Curves 22 and 23 corresponding to a volume fraction of the coreV/V_(c)=0.75 give, of course, an intermediate result.

What is claimed is:
 1. Process for obtaining thin, lightweight and rigidmetal parts, said process comprising the following steps: producing athin core made of a metal alloy; applying, to each face of the core, ashell consisting of reinforcing fibers having an elastic modulus atleast equal to four times that of the metal alloy, said reinforcingfibers being embedded in a matrix made of a matrix-forming metal alloy;densifying the shells by pressing at least in the thickness direction ata superplasticity temperature of the metal alloy of the matrix; anddiffusion-bonding of said shells to said core by pressing in thethickness direction of the part at an isothermal forging temperature ofthe metal alloy of the core and of the matrix of the shells; wherein aplurality of cavities is produced in the core and emerge at least on oneface of the core; and wherein the shells are densified to the shape ofthe core.
 2. Process according to claim 1, wherein a metal alloy,selected from a group consisting of titanium-, magnesium- andaluminum-based alloys, is employed; and wherein the reinforcing fibers,selected from a group consisting of silicon carbide, boron and carbon,are employed, so as to combine a light metal alloy with reinforcingfibers having a high strength and a high elastic modulus.
 3. Processaccording to claim 1 or 2, wherein the diffusion-bonding step is carriedout by pressing in a die in a press.
 4. Process according to claim 1 or2, wherein the diffusion-bonding step is carried out by isostaticpressing in an autoclave.
 5. Process according to claim 1 or 2, whereina volume fraction of the core is increased in fastening regions of thepart so as to increase compressive strength of the part in the fasteningregions.
 6. Process according to claim 5, wherein the core is producedto extend into a blade and into a root of the part and wherein thecavities are produced in the core of the blade with a reduced volumefraction of the core, said part being a turbomachine blade assembly. 7.Process according to claim 6, wherein the blade assembly is made in adie.
 8. Process according to claim 1 or 2, wherein the cavities areproduced in the core along lines so as to form ducts between the shells.9. Process according to claim 8, wherein the cavities each emerge onlyon one side of the core, so as to maintain cohesion of said core duringproduction of the part.